This invention relates to highly soluble poly(borazylenes), to a direct thermal process, not requiring a catalyst, for preparing poly(borazylenes), and to the use of such poly(borazylenes) as processable precursors for boron nitride. This invention further relates to borazine/polyhedral borane oligomers which can also be made by a direct thermal process without a catalyst.
Interest in the development of ceramic/ceramic composite materials stems from a desire to improve structural integrity over that of a single ceramic component. For example, ceramic fiber reinforced ceramics are known to exhibit increased strength and toughness due to a lessening of crack propagation. Pipes, B. R., McCullough, R. L., Chou, T. W., Scientific American, 1986, 193-203; Bracke, P., Schurmans, H., Vehoest, J., "Inorganic Fibers and Composite Materials" EPO Applied Technology Series Volume 3, Pergamon, New York, 1984. A suitable ceramic fiber coating can enhance the strength of a ceramic fiber/ceramic composite by decreasing the interfacial shear strength between the fiber and matrix and thus increase the potential for fiber pullout (toughness). Another benefit of fiber coatings is that they may serve as a diffusion barrier between fibers and matrix materials and, thus, inhibit chemical reactions between these materials at high temperatures. Boron nitride (BN) is a non-oxide ceramic which because of its excellent strength and chemical resistance is an attractive prospect as a ceramic coating for fibers in ceramic fiber/ceramic composites.
Previous methods for the formation of coatings or thin films of BN have generally relied on the use of vapor deposition (CVD) techniques, employing mixtures of NH.sub.3 and volatile borane species such as BCl.sub.3, B.sub.2 H.sub.6 and B.sub.3 N.sub.3 H.sub.6. Gmelin Handbuch der Anorganishen Chemie, Boron Compounds, 1980, Third Supplement, Vol.3, Sec 4 and references therein, and 1988, 3rd Supplement, Volume 3. For example, conventional CVD techniques have been used for the preparation of thin films of BN from a BCl.sub.3 -NH.sub.3 -H.sub.2 mixture at 1000.degree.-1400.degree. C., while plasma assisted CVD of a B.sub.2 H.sub.6 -NH.sub.3 -H.sub.2 mixture results in a deposition of a thin layer of BN in the temperature range of 400.degree.-700.degree. C. Lowden, R. A., Besmann, T. M., Stinton, D. P., Ceram. Bull. 1988, 67, 350-355. Although the CVD technique offers an effective pathway for depositing a uniform layer of a ceramic on a variety of substrates, these procedures are often time consuming and costly. An alternative method for generating BN coatings could employ a coatable, non-volatile chemical precursor which could be thermally decomposed to BN on a desired substrate. Indeed, several boron based polymer systems displaying this set of properties have been developed as potential precursors to BN coatings. Paine, R. T., Narula, C. K., Chem. Rev. 1990, 90, 73-92 and references therein; Narula, C. K., Schaeffer, R., Paine, R. T., J. Am. Cer. Soc. 1987, 109, 5556-5557; Narula, C. K., Paine, R. T., Schaeffer, R., Polymer Prep, (Am. Chem. Soc. Div. Polym. Chem.) 1987, 28, 454; Narula, C. K., Paine, R. T., Schaeffer, R. in Better Ceramics Through Chemistry II, Brinker, C. J., Clark, D. E., Ulrich, D. R. Eds, MRS Symposium Proceedings 73, Materials Research Society:Pittsburgh Pa., 1986, 363-388; Narula, C. K., Paine, R. T., Schaeffer, R., in Inorganic and Organometallic Polymers, Zeldin, M., Wynne, K. J., Allcock, H. S. Eds., ACS Symposium Series 360, American Chemical Society: Washington, D.C. 1988, 378-384; Paciorek, K. J. L., Harris, D. H., Krone-Schmidt, W., Kratzer, R. H., Technical Report No. 4, Ultrasystems Defense and Space Inc., Irvine, Calif. 1978; Paciorek, K. J. L., Krone-Schmidt, W., Harris, D. H., Kratzer, R. H., Wynne, K. J. in Inorganic and Organometallic Polymers, Zeldin, M., Wynne, K. S., Allcock, H. S., Eds., ACS Symposium Series 360, American Chemical Society: Washington, D.C. 1988, 27, 3271; Rees, W. S., Seyferth, D., presented at the 194th National Meeting of the American Chemical Society, New Orleans, La., Sept. 1987, Paper INOR 446; Rees, W. S., Jr., Seyferth, D., J. Am. Ceram. Soc., 1988, 71, C194-C196; Mirabelli, M. G. L., Sneddon, L. G., Inorg. Chem. 1988, 27, 3721; Mirabelli, M. G. L., Lynch, A. T., Sneddon, L. G., Solid State Ionics, 1989, 32/33,655-660; Lynch, A. T., Sneddon, L. G., J. Am. Chem. Soc., 1989, 111, 6201-6209.
Poly(borazylenes), polymers comprising linked borazine rings analogous to organic poly(phenylenes), would be ideal precursors for BN if they could be prepared in high yield and had high enough solubility. Small dehydrodimers and oligomers of alkylated borazine have previously been prepared, primarily either by metathesis or coupling reactions; however, owing to its greater reactivity these procedures are unsuitable for the generation of analogous species based on the parent B.sub.3 N.sub.3 H.sub.6 compound. Wagner, R. I., Bradford, J. L., Inorg. Chem. 1962, 99-106; Brotherton, R. J., McCloskey, A. L., U.S. Pat. No. 3,101,369, 1963, Chem. Abstr. 1964, 60, 547; Gutman, V., Meller, V., Schlegel, R., Monatsh. Chem. 1964, 95, 314-318; Gerrard, W., Hudson, H. R., Mooney, E. F., J. Chem Soc. 1962, 113-119; Harris, J. J., J. Org. Chem. 1961, 26, 2155-2156. The N-B coupled dimer 1:2'- B.sub.3 N.sub.3 H.sub.5 !.sub.2 has been obtained in low yields from the decomposition of liquid borazine at room temperature over several months (Manatov, G., Margrave, J. L., J. Inorg. Nucl. Chem. 1961, 20, 348-351) and from the gas phase photolytic (Neiss, M. A., Porter, R. F., J. Am. Chem. Soc. 1972, 94, 1438-1443) or pyrolytic (Laubengayer, A. W., Moews, P. C., Jr., Porter, R. F., J. Am. Chem. Soc. 1961, 83, 1337-1342) reactions of borazine. The latter two studies also reported the formation of insoluble solids that were proposed to have fused borazine polycyclic structures. Several studies of the stability of liquid borazine have also reported the formation of white low volatile solids, but these materials were not identified. Manatov, G., et al., Op cit.. Schaeffer, R., Steindler, M., Hohnstedt, L., Smith, H. R., Jr., Eddy, L. B., Schlesinger, H. I., J. Am. Chem. Soc. 1954, 76, 3303-3306; Haworth, D. T., Hohnstedt, L. F., J. Am. Chem. Soc. 1960, 82, 3860-3862.
The preparation of certain polyborazine BN-precursors, in which borazine rings are linked by a bridging nitrogen atom, are disclosed in U.S. Pat. No. 4,801,439 (Blum et al.). Blum et al. disclose that compounds containing at least one Group IIIA metal-Group VA nonmetal bond can be prepared by reacting a first reactant having at least one ZH bond where Z represents a Group. VA nonmetal with a second reactant that has at least one M-H bond where M is a Group IIIA metal in the presence of a metal catalyst. The metal catalysts disclosed by Blum et al. for use in their method include: homogeneous catalysts such as H.sub.4 Ru.sub.4 (CO).sub.12, Ru.sub.3 (CO).sub.12, Fe.sub.3 (CO).sub.12, Rh.sub.6 (CO).sub.16, Co.sub.2 (CO).sub.8, (Ph.sub.3 P).sub.2 Rh(CO)H, H.sub.2 PtCl.sub.6, nickel cyclooctadiene, Os.sub.3 (CO).sub.12, Ir.sub.4 (CO).sub.12, PdCl.sub.2, (PhCN).sub.2 PdCl.sub.2, (Ph.sub.3 P).sub.2 Ir(CO)H, Pd(OAc).sub.2, CP.sub.2 TiCl.sub.2, (Ph.sub.3 P).sub.3 RhCl, H.sub.2 Os.sub.3 (CO).sub.10, Pd(Ph.sub.3 P).sub.4, Fe.sub. 3 (CO).sub.12 /Ru.sub.3 (CO).sub.12 complexes of metal hydrides, and heterogeneous catalysts such as alkaline metals (e.g., Na, K), Pt/C, Pt/BaSO.sub.4, Cr, Pd/C, Co/C, Pt black, Co black, Pd black, Ir/Al.sub.2 O.sub.3, Pt/SiO.sub.2, Rh/TiO.sub.2, Rh/La.sub.2 O.sub.3, Pd/Ag alloy, LaNi.sub.5, PtO.sub.2, tranasition metal salts, transition metal hydrides or other transition metal oxides. It is disclosed that either the first reactant or the second reactant, or both, may be borazine. Comparative examples presented in the patent show that preparations carried out in the absence of metal catalyst either did not produce the desired product or produced no product at all. No examples are given of the preparation of a poly(borazylene) polymer.
In Lynch, A. T., Sneddon, L. G., Abstracts of Papers of American Chemical Society Meeting, Los Angeles, Calif., 1988, paper No. 296, the polymerization of borazine in the presence of CpTiMe.sub.2 catalyst was reported. The resulting poly(borazylene), after recrystallization, cannot be totally redissolved in organic solvents such as THF or glyme. Further, upon standing for extended periods of time, e.g., at least about one week, the poly(borazylene) prepared using metal catalyst becomes totally insoluble in solvents such as ethers and glyme. Since a processable (i.e., soluble) poly(borazylene) is desired for use as a BN ceramic precursor, the material prepared according to the Lynch et al. disclosure has obvious disadvantages.
Transition metal promoted reactions producing coupled products of borazine and polyhedral boranes have been disclosed. A. T. Lynch, Ph.D. Thesis, University of Pennsylvania, 1989. However, there have been no reports of a simple thermolytic route to these species.
Swiss Patent 670 105, published Dec. 5, 1989, discloses a photolytic method for making dimers of borazine which can be used to make BN coatings. These dimeric mixtures are composed of diborazine and borazanaphthalene or mixtures of the two. The patent suggests that reaction times can be shortened by using transition metal catalysts.